1. Field of the Invention
This invention relates to a process for the production of granular quartz glass by the thermal treatment of silica gels obtained by hydrolysis of orthosilicic acid esters of aliphatic alcohols. More especially this invention relates to the preparation of fused silica, i.e., pure silicate glass by a process involving the hydrolysis of a silicic acid ester obtained by reaction of silicon or ferrosilicon with a C.sub.1 -C.sub.2 aliphatic alcohol, with an excess amount of water whereby to prepare a silica sol, effecting gelation of the silica sol and preparation of fused silica quartz glass therefrom by heating at increasing temperatures up to 1400.degree. C.
2. Discussion of the Prior Art
The subject of the invention is a process for the production of granular quartz glass by the thermal treatment of silica gels obtained by a hydrolytic method from orthosilicic acid esters.
It is known to produce quartz glass by melting rock crystal of natural origin. Since rock crystal, as a natural product, contains impurities, such as, for example, iron, titanium and aluminum compounds and calcium, magnesium and alkali compounds, it must be refined, especially if it is to be used for optical purposes. Even the types of rock crystal which are considered to be of high purity cannot be used for the production of fused quartz glass without complicated purification processes. Even small amounts of impurities, such as iron, titanium and other metals in amounts of a few parts per million will impair transparency to ultraviolet rays. Also undesirable is the hydroxyl group content, which produces an absorption in the infrared range.
Therefore, many attempts have been made to produce quartz glasses by synthetic methods.
For example, it has been attempted to obtain quartz glass by melting precipitated silicic acid gels, but this yields melts which are bubbly.
For the prevention of bubbles it is proposed in German Pat. No. 1,596,473 to treat precipitated silicic acids with silicon tetrachloride at elevated temperature, and thus to eliminate the residual water which produces the bubbles. This known procedure is difficult and expensive. The use of silicon tetrachloride requires especially designed apparatus to prevent corrosion and environmental pollution. It is also especially disadvantageous that the precipitated silicic acid cannot be prepared with the required high purity.
It has also been proposed to produce high-purity silicic acid gel from tetraethoxysilane, and transform it to silicon dioxide by sintering at 650.degree. to 750.degree. C. (Che. Inc., USSR, 1969, No. 6, pp 47 (447)-49 (449)). Such a product is said to be able to be melted to form bubble-free quartz glass. The disadvantage of this known method lies, on the one hand, in the difficult purification of the ester, and, on the other, in the low yield of silicic acid gel due to the great excess of water, and in the long gelling times. Since the tetraethoxysilane is prepared from silicon tetrachloride, it is contaminated by various elements in the form of chlorides. It is therefore proposed in this publication to purify the ester first by combined treatment with chelating reagents and active carbon, and then to complete the purification by the absorptive capacity of the silicic acid in statu nascendi with partial hydrolysis of the ester. The ester is then reacted in the acid medium to form silicic acid sol, and brought to gelation by the addition of ammonia water. The hydrolysis of the ester is performed with a 24-fold excess of water.
It is also known to react silicon tetrachloride with oxygen at elevated temperatures to form silicon dioxide, and to use the silicon dioxide as the starting product for the preparation of fused quartz glass. This process, however, is very expensive and difficult to perform technically on account of the formation of chlorine.
It is an object of this invention, therefore, to provide a process by which bubble-free, high-purity quartz glass is produced, and which at the same time does not have the disadvantages of the prior art processes.